Amorphous silicon film, its production and photo semiconductor device utilizing such a film

ABSTRACT

An amorphous silicon film contains not less than 30 at. % hydrogen and includes silicon atoms joined with one hydrogen atom and silicon atoms joined with two hydrogen atoms, the ratio of the silicon atoms joined with two hydrogen atoms to the silicon atoms joined with one hydrogen atom being not more than 0.4. This amorphous silicon films is produced by performing plasma-assisted chemical vapor deposition at a substrate temperature of not more than 100° C., while supplying hydrogen and silane in the predetermined ratio, the ratio of the flow of hydrogen to that of silane being not less than 1.

This is a division of application Ser. No. 574,019 filed Aug. 29, 1990,now U.S. Pat. No. 5,152,833.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an amorphous silicon film and itsproduction. More particularly, the present invention relates to anamorphous silicon film with wide optical bandgap and high photoelectricconductivity and a method for production of such a film. Also, thepresent invention relates to a photo-semiconductor device utilizing suchan amorphous silicon film.

2. Description of the Prior art

In general, amorphous silicon films for use in photo semiconductordevices such as, for example, photovoltaic devices are required to havea wide bandgap to use incident light effectively.

To this end, so far, it has been proposed to incorporate additives suchas carbon or oxygen into amorphous silicon films. However, incorporationof such an additive cause decrease in characteristics of the film,especially, photo electric conductivity, as disclosed in Journal ofNon-Crystalline Solids 97&98 (1987) 1027-1034.

On the other hand, amorphous silicon films with a wide bandgap can beproduced by increasing the hydrogen concentration in the amorphoussilicon film, without incorporation of additives such as carbon. In thiscase, it is possible to produce amorphous silicon films containing 30atomic % of hydrogen if the substrate is maintained at a temperature of100° C. or less during formation of the film. However, if the substratetemperature is not more than 100° C., the photo electric conductivity ofthe film decreases considerably, thus making it impossible to produceamorphous silicon films with excellent characteristics.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide amorphoussilicon films having a wide bandgap and excellent film characteristics,especially, high photo electric conductivity.

Another object of the present invention is to provide a method forproducing amorphous silicon films having a wide bandgap and high photoelectric conductivity.

Still another object of the present invention is to provide a photosemiconductor devices such as photovoltaic devices comprising anamorphous silicon film with a wide bandgap and high photo electricconductivity.

According to the present invention, these and other objects are solvedby providing an amorphous silicon film containing not less than 30 at. %hydrogen and including silicon atoms joined with one hydrogen atom andsilicon atoms joined with two hydrogen atoms, the ratio of said siliconatoms joined with two hydrogen atoms to silicon atoms joined with onehydrogen atom being not more than 0.4.

According to the present invention, the above amorphous silicon film maybe produced by a method in which plasma-assisted chemical vapordeposition (PACVD) is carried out at a substrate temperature of not morethan 100 ° C., while supplying hydrogen (H₂) gas and silane (SiH₄) gasin a suitable flow ratio into the reaction system, the flow ratio ofhydrogen gas to silane gas being not less than 1.

The PACVD process includes the steps of positioning a substrate betweena pair of electrodes in a reaction vessel, evacuating the reactionvessel to a vacuum of 10⁻⁵ Torr or above, introducing hydrogen andsilane in the predetermined ratio into the reaction vessel to adjust apressure in the vessel to 0.2 Torr or less, and supplying a radiofrequency power between the electrodes, while supplying hydrogen andsilane in the above ratio to the vessel. During the plasma-assistedchemical vapor deposition, the pressure in the vessel is kept constantby evacuation.

In a preferred embodiment, the above amorphous silicon film is producedby the PACVD process in which a radio frequency (RF) power of not morethan 17 mW/cm² is

to the electrodes at a substrate temperature of not more than 100 ° C.

In another preferred embodiment, the amorphous silicon film is producedby the PACVD process at a substrate temperature of not more than 100° C.and at a reaction pressure of not more than 0.1 Torr.

The amorphous silicon film according to the present invention has a widebandgap because of its high concentration of hydrogen atoms of not lessthan 30 at. %. In addition, the film of the present invention possesseshigh photo-conductivity as it contains silicon atoms joined with onehydrogen atom (hereinafter referred to as a SiH bond) and silicon atomsjoined with two hydrogen atoms (hereinafter referred to as SiH₂ bond) inthe SiH₂ bond / SiH bond ratio of not more than 0.4.

According to the present invention, there is also provided aphoto-semiconductor device comprising an amorphous silicon filmcontaining not less than 30 at. % hydrogen atoms and including siliconatoms joined with one hydrogen atom and silicon atoms joined with twohydrogen atoms in the ratio of the SiH₂ bond to the SiH bond being notmore than 0.4.

The invention will be further apparent from the following descriptionwith reference to several examples and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing variation of the ratio of silicon atoms joinedwith one hydrogen atom to silicon atoms joined with two hydrogen atomsin the amorphous silicon film with the flow ratio of hydrogen (H₂) tosilane (SiH₄) during fabrication of the film;

FIG. 2 is a graph showing variation of photo electric conductivity withthe ratio of the number of silicon atoms joined with two hydrogen atomsto that of silicon atoms joined with one hydrogen atom in the amorphoussilicon film;

FIG. 3 is a graph showing variation of photo electric conductivity (σph)with temperature for amorphous silicon films of the present inventionand the prior art;

FIG. 4 is a graph showing the relationship between the photo electricconductivity and RF power;

FIG. 5 is a graph showing the relationship between the ratio of thenumber of silicon atoms joined with two hydrogen atoms to that ofsilicon atoms joined with one hydrogen atom in the amorphous siliconfilm and RF power;

FIG. 6 is a graph showing the relationship between the photo electricconductivity and the gas pressure in the reaction system;

FIG. 7 is a graph showing the relationship between the gas pressure inthe system and the ratio of the number of silicon atoms joined with twohydrogen atoms to that of silicon atoms joined with one hydrogen atom inthe film produced.

FIGS. 8 to 11 are schematic sectional views showing variousphoto-semiconductor devices according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Example 1

Using the known parallel-plate reactor for PACVD, amorphous siliconfilms were produced by plasma-assisted chemical vapor deposition fromsilane (SiH₄) and hydrogen (H₂). The PACVD was carried out in thefollowing manner: After positioning a substrate between a pair ofelectrodes, a reaction vessel is evacuated to a vacuum of more than1×10⁻⁶ Torr, and then supplied with hydrogen and silane in thepredetermined ratio to adjust the pressure in the vessel to 0.2 Torr. Atthe same time, the substrate is heated to a temperature of not more than100° C. While supplying hydrogen and silane in the above ratio andkeeping the pressure in the vessel constant, a radio frequency power isapplied between the electrodes to carry out plasma-assisted chemicalvapor deposition under the following conditions.

    ______________________________________                                        Substrate temperature:                                                                            80° C.                                             RF power (13.56 MHz):                                                                             25 W (25 mW/cm.sup.2)                                     Pressure:           0.2 Torr                                                  Flow of SiH.sub.4 : 40 sccm                                                   Flow of H.sub.2 :   0-200 sccm                                                ______________________________________                                    

The quantity of the SiH bond and that of the SiH₂ bond in the depositedfilm was determined by measurement of peak values of infrared spectrumor Raman spectrum at 2000 cm⁻¹ and 2090 cm⁻¹. The ratio of the SiH₂ bondto the SiH bond, i.e., the SiH₂ bond/SiH bond ratio, are plotted in FIG.1 as a function of the ratio of the flow of H₂ to that of SiH₄ duringdeposition.

The above amorphous silicon film was provided with a spaced pair ofelectrodes with size of 2 cm×2 cm to determine the photo electricconductivity. The photo electric conductivity (σph) of each amorphoussilicon film was determined by measurement of a current flowing underexposure to light with a strength of 100 mW/cm². The results are plottedin FIG. 2 as a function of the SiH₂ bond/SiH bond ratio.

As will be understood from the results shown in FIG. 1, the SiH₂bond/SiH bond ratio in the deposited film can be controlled within therange from about 0.1 to 1 by adjusting the ratio of the flow of H₂ tothat of SiH₄. Further, as can be seen from the results shown in FIG. 2,the photoelectric conductivity (σph) varies with the SiH₂ bond/SiH bondratio and exceeds 10⁻⁵ /Ωcm if the SiH₂ bond/SiH bond ratio in thedeposited film is not greater than 0.4.

For these reasons, the composition of the amorphous silicon film of thepresent invention has been limited to those containing not less than 30at. % of hydrogen and including silicon atoms joined with two hydrogenatoms and silicon atoms joined with one hydrogen atom in the SiH₂bond/SiH bond ratio of not more than 0.4.

Example 2

There were prepared amorphous silicon films in the same manner asExample 1 except for that the flow amount of hydrogen gas is adjusted to40 sccm or 200 sccm to that the flow rate of H₂ to SiH₄ is 1 : 1, or 5 :1.

Comparative Example 1

There were prepared amorphous silicon films in the same manner asExample 1 except for that no hydrogen gas was supplied to the reactionsystem.

The amorphous silicon films prepared in example 2 and comparativeexample 1 were subjected to measurement of the photo electricconductivity. The measurements were made just after production of thefilm, after leaving the film at room temperature for more than 1 month,and after annealing it at 80° C. or 160° C. for 2 hours. Results areplotted in FIG. 3.

From the results shown in FIG. 3, it will be seen that the photoelectricconductivity of the amorphous silicon films of the present inventionscarcely varies with the temperature. Thus, the amorphous silicon filmof the present invention possesses high heat resistivity.

Example 3

There were prepared amorphous silicon films in the same manner asExample 1 except for that the RF power supplied was adjusted to a valuein the range of 15 to 25 mW/cm². Other conditions for PACVD were asfollows:

    ______________________________________                                        Substrate temperature:                                                                            80° C.                                             Flow of silane:     10 sccm or 40 sccm                                        Flow of hydrogen:   0 sccm                                                    ______________________________________                                    

The resultant amorphous silicon films were subjected to measurements ofthe photo electric conductivity (σph) and the SiH₂ bond/SiH bond ratioin the deposited film. Results are plotted in FIGS. 4 and 5 as afunction of the RF power, respectively.

As can be seen from the results shown in FIGS. 4 and 5, when the RFpower is not more than 17 mW/cm², the SiH₂ bond/ SiH bond ratio in thedeposited film becomes not more than 0.4, and the photo electricconductivity σph) becomes not less than 10⁻⁷ /Ωcm, regardless of theflow of silane during deposition.

Example 4

There were prepared amorphous silicon films in the same manner asExample 1 except for that the pressure in the reaction system wasadjusted to the predetermined value of not more than 0.2 Torr. Theconditions other than the gas pressure are as follows:

    ______________________________________                                        Substrate temperature:                                                                            80° C.                                             RF power (13.56 MHz):                                                                             25 W (25 mW/cm.sup.2)                                     Flow of SiH.sub.4 : 10 sccm or 40 sccm                                        Flow of H.sub.2 :   0 sccm                                                    ______________________________________                                    

For each resultant amorphous film, measurements were made on photoelectric conductivity (σph) and the SiH₂ bond/SiH bond ratio in thedeposited film. Results are plotted in FIGS. 6 and 7 as a function ofpressure in the reaction system, respectively.

As can be seen from the results shown in FIGS. 6 and 7, when theamorphous silicon film is produced under the reaction pressure of notmore than 0.1 Torr, the SiH₂ bond / SiH bond ratio becomes not more than0.4, and the photo electric conductivity (σph) becomes not less than10⁻⁷ /Ωcm, regardless of the flow of silane during deposition.

Thus, it is possible to produce amorphous silicon films which have ahigh bandgap of 1.9 to 2.0 eV and high electric conductivity of 10⁻⁷/Ωcm to 10⁻⁵ /Ωcm by carrying out the PACVD process on condition thatthe substrate temperature is not more than 100 ° C. and the reactionpressure is not more than 0.1 Torr.

Referring now to FIG. 8, there is shown a photovoltaic device which isone embodiment of a photo semiconductor device utilizing an amorphoussilicon film of the present invention. This device comprises a glasssubstrate 1, a transparent electrode layer 2 composed of a SnO₂ filmwith a thickness of 5000 Å, p-type layer 3 composed of a B-dopedamorphous silicon carbide film with a thickness of 100 Å, i-type layer 4composed of an amorphous silicon film with a thickness of 5000 Å, n-typelayer 5 composed of a P-doped amorphous silicon film with a thickness of500 Å, and an electrode layer 6 composed of a metal such as Ag. Theselayers are deposited in the order illustrated.

The amorphous silicon film used as the i-type layer 4 is formed, inaccordance with the present invention, by the PACVD process under thefollowing conditions:

    ______________________________________                                        Substrate temperature:                                                                            80° C.                                             RF power (13.56 MHz):                                                                             35 W (35 mW/cm.sup.2)                                     Reaction pressure:  0.2 Torr                                                  Flow of SiH.sub.4 : 10 sccm                                                   Flow of H.sub.2 :   20 sccm                                                   ______________________________________                                    

It was observed that the concentration of hydrogen in the producedamorphous silicon film is 35 at. %, and the SiH₂ bond / SiH bond ratiois 0.3. An open-circuit voltage of the photovoltaic device comprisingsuch an amorphous silicon film was 0.95 V, whereas that of thephotovoltaic device comprising the amorphous silicon film of the priorart was 0.85 V.

In the above embodiment, the B-doped amorphous silicon carbide film hasbeen used as the p-type layer 3. This may be replaced with a B-dopedamorphous silicon film produced under the following conditions:

    ______________________________________                                        Substrate temperature: 80° C.                                          RF power (13.56 MHz):  25 W                                                   Reaction pressure:     0.2 Torr                                               Flow of SiH.sub.4 :    10 sccm                                                Flow of H.sub.2 :      20 sccm                                                Flow of B(CH.sub.3).sub.3 :                                                                          0.3 sccm                                               ______________________________________                                    

If such an amorphous silicon film is used as the p-type layer 3, thephoto absorption coefficient based on a wide bandgap of the film isreduced to one-half the value (1×10⁴ cm⁻¹ at 650 nm) of the photovoltaicdevice of the prior art. Thus, it is possible to reduce the absorptionloss caused by the p-type layer 3. This is supported by the fact thatthe short circuit current of the photovoltaic device is increased from17 mA/cm² to 18 mA/cm².

Also, the P-doped amorphous silicon film used as the n-type layer 5 maybe replaced with a P-doped amorphous silicon film produced under thefollowing conditions:

    ______________________________________                                        Substrate temperature: 80° C.                                          RF power (13.56 MHz):  25 W                                                   Reaction pressure:     0.2 Torr                                               Flow of SiH.sub.4 :    10 sccm                                                Flow of H.sub.2 :      20 sccm                                                Flow of PH.sub.3 :     0.1 sccm                                               ______________________________________                                    

It was observed that, when the above P-doped amorphous silicon film isused for the n-type layer 5, the curve factor of the photovoltaic deviceis improved from 0.70 to 0.75 because of its high photo electricconductivity (σph) of not less than 10⁻⁵ /Ωcm.

FIG. 9 shows a photovoltaic device which is another embodiment of aphoto semiconductor device utilizing an amorphous silicon film of thepresent invention. In this figure, the same parts are denoted by thesame reference numerals with those of FIG. 8. The photovoltaic device ofFIG. 9 has the same structure as that of the photovoltaic device shownin FIG. 8, except for that two i-type layers, i.e., first i-type layer 7and second i-type layer 8, are formed between the p-type layer 3 andn-type layer 5 in the photovoltaic device of FIG. 8. The first i-typelayer 7 is composed of an amorphous silicon film with a 500 Å, whereasthe second i-type layer 8 is composed of an amorphous silicon film witha bandgap of not more than 1.9 eV and a thickness of 5000 Å.

The amorphous silicon layer for the first i-type layer 7 was formed bythe PACVD process in accordance with the present invention under thefollowing conditions:

    ______________________________________                                        Substrate temperature:                                                                            80° C.                                             RF power (13.56 MHz):                                                                             25 W (25 mW/cm.sup.2)                                     Reaction pressure:  0.2 Torr                                                  Flow of SiH.sub.4 : 10 sccm                                                   Flow of H.sub.2 :   20 sccm                                                   ______________________________________                                    

The second i-type layer 8 was formed by the PACVD process under thefollowing conditions:

    ______________________________________                                        Substrate temperature: 200° C.                                         RF power (13.56 MHz):  25 W                                                   Reaction pressure:     0.2 Torr                                               Flow of SiH.sub.4 :    10 sccm                                                ______________________________________                                    

No hydrogen was supplied to the reaction system.

For the photovoltaic device produced, the measurements were made onphotoelectric properties including open-circuit voltage, short-circuitcurrent, and conversion efficiency. Results are as follows:

    ______________________________________                                        Open-circuit voltage: 0.91 V                                                  Short-circuit current:                                                                              17.5 mA/cm.sup.2                                        Conversion efficiency:                                                                              11.2%                                                   ______________________________________                                    

The photovoltaic device of this embodiment is much improved in thephotoelectric properties as the photovoltaic device of the prior artpossesses the open-circuit voltage of 0.85 V, short-circuit current of17 mA/cm² and conversion efficiency of 10.4% even at the maximum. Theprovision of the first i-type layer 7 contributes to improve theadhesion between the p-type layer and the second i-type layersubstantially serving as the photo absorption layer, and allows thelight to transmit therethrough to the second i-type layer.

The amorphous silicon film for the first i-type layer 7 may be replacedwith an amorphous silicon film having a thickness of 200 Å and beingproduced under the following conditions:

    ______________________________________                                        Substrate temperature: 50° C.                                          RF power (13.56 MHz):  15 W                                                   Reaction pressure:     0.1 Torr                                               Flow of SiH.sub.4 :    10 sccm                                                ______________________________________                                    

FIG. 10 shows a photo sensor which is another example of a photosemiconductor device utilizing an amorphous silicon film of the presentinvention. The photo sensor comprises a substrate 10 of quartz, atransparent electrode film 11 of SnO₂ with a thickness of 1000 Å, aninsulating film 12 of amorphous silicon nitride with a thickness of 100Å, an i-type layer 13 composed of an amorphous silicon film with athickness of 300 Å, an n-type layer 14 composed of a P-doped amorphoussilicon film with a thickness of 500 Å, and an electrode film 15composed of a metal such as Al. These layers or films are deposited inthe order illustrated.

The i-type layer 13 is composed of an amorphous silicon film of thepresent invention, which is formed by the PACVD process under thefollowing conditions:

    ______________________________________                                        Substrate temperature:                                                                            30° C.                                             RF power (13.56 MHz):                                                                             50 W (50 mW/cm.sup.2)                                     Reaction pressure:  0.1 Torr                                                  Flow of SiH.sub.4 : 10 sccm                                                   Flow of H.sub.2 :   100 sccm                                                  ______________________________________                                    

The photo sensor with the above construction is reduced in the photoabsorption coefficient by about 70%, as compared with that of thephotovoltaic device of the prior art, i.e, 1×10⁴ cm⁻¹ at 650 nm, sincethe amorphous silicon film of the present invention has a wide bandgap.The amorphous silicon film of the present invention allow light with awavelength of 500 nm to transmit not less than 95%, thus making itpossible to produce photo cell responsible to the light of not less than500 nm, without use of any specific optical filters.

FIG. 11 shows a cross section of a photosensitive drum used forelectrophotography, which is another example of a photo semiconductordevice utilizing an amorphous silicon film of the present invention. Thephotosensitive drum comprises a cylindrical substrate 20 of aluminum, abarrier layer 21 deposited on the substrate 20 and composed of anamorphous silicon film with a thickness of 2 μm, a photoconductive layer22 deposited on the barrier layer 21 and composed of an amorphoussilicon layer with a thickness of 25 μm, and a surface layer 23deposited on the photoconductive layer 22 and composed of an amorphoussilicon carbide film with a thickness of 1 μm.

In this embodiment, the photoconductive layer 22 is formed by the PACVDprocess of the present invention. However, other layers may be depositedby any known thin film deposition techniques such as chemical vapordeposition, vacuum evaporation, sputtering, etc.

The photoconductive layer 22 is prepared by depositing an amorphoussilicon film of the present invention on the barrier layer 21, i.e., bythe PACVD process of the present invention under the followingconditions:

    ______________________________________                                        Substrate temperature:                                                                           100° C.                                             RF power (13.56 MHz):                                                                            100 W (35 mW/cm.sup.2)                                     Reaction pressure: 0.2 Torr                                                   Flow of SiH.sub.4 :                                                                              50 sccm                                                    Flow of H.sub.2 :  50 sccm                                                    ______________________________________                                    

The photoconductive layer prepared under such conditions showed that theresistivity is 10¹¹ Ωcm in the dark but is reduced to 10⁵ Ωcm whenexposed to light. Thus, the ratio of the dark resistivity to theresistivity under exposure to light is 10⁶, which is greater than thatof the photoconductive layer of the prior art since the resistivity ofthe latter is 10¹¹ Ωcm in the dark but 10⁶ Ωcm under exposure to light.Thus, the photoconductive layer of the amorphous silicon film of thepresent invention makes it possible to improve the image contrast.

What is claimed is:
 1. A photo-semiconductor device comprising aplurality of films on a substrate, at least one of said films being anamorphous silicon film consisting essentially of silicon and not lessthan 30 at. % hydrogen and including silicon atoms joined with onehydrogen atom and silicon atoms joined with two hydrogen atoms, theratio of said silicon atoms joined with two hydrogen atoms to saidsilicon atoms joined with one hydrogen atom being not more than 0.4.